1. Field of the Invention
This invention relates broadly to fiber optic temperature sensing systems. More particularly, this invention relates to fiber optic distributed temperature sensing systems based on optical time-domain reflectometry.
2. Description of Related Art
Fiber optic distributed temperature sensing (DTS) systems are generally based on optical time-domain reflectometry (OTDR), which is commonly referred to as “backscatter.” In this technique as shown in prior art FIG. 1, a pulsed-mode high power laser source 1 launches a pulse of light along an optical fiber 2 through a directional coupler 3. The optical fiber 2 forms the temperature sensing element of the system and is deployed where the temperature is to be measured. This may be along power cables, tunnels, pipelines, oil wells, or other structures. As the pulse propagates along the optical fiber 2 its light is scattered through several mechanisms, including density and composition fluctuations (Rayleigh scattering) as well as molecular and bulk vibrations (Raman and Brillouin scattering, respectively). Some of this scattered light is retained within the fiber core and is guided back towards the source 1. This returning signal is split off by the directional coupler 3 and sent to a highly sensitive receiver 4. In a uniform fiber, the intensity of the returned light shows an exponential decay with time (and reveals the distance the light traveled down the fiber based on the speed of light in the fiber). Variations in such factors as composition and temperature along the length of the fiber show up in deviations from the “perfect” exponential decay of intensity with distance, as shown in the graph of prior art FIG. 2.
The OTDR technique is well established and used extensively in the optical telecommunications industry for qualification of a fiber link or fault location. In such an application, the Rayleigh backscatter signature is examined. As shown in prior art FIG. 3, the Rayleigh backscatter signature is unshifted from the launch wavelength. This signature provides information on loss, breaks, and inhomogeneities along the length of the fiber; and it is very weakly sensitive to temperature differences along the fiber. The two other backscatter components (the Brillouin backscatter signature and the Raman backscatter signature) are shifted from the launch wavelength and the intensity of these signals are much lower than the Rayleigh component as shown in FIG. 3. The Brillouin backscatter signature and the “Anti-Stokes” Raman backscatter signature are temperature sensitive. Either one (or both) of these backscatter signatures can be extracted from the returning signals by the optical filter 5 and detected by detector 6 as shown in prior art FIG. 1. The detected signals are processed by the signal processing circuitry 7, which typically amplifies the detected signals and then converts (e.g., digitized by a high speed analog-to-digital converter) the resultant signals into digital form. The digital signals may then be analyzed to generate a temperature profile along the optical fiber.
The measure of the ability of a DTS system to resolve adjacent temperature features along the length of the optical fiber is known as “spatial resolution” and is critically dependent on the width of the optical pulse. The measure of the ability of the instrument to resolve the temperature of a particular feature is known as “temperature resolution.” This is dependent on the signal-to-noise ratio of the received signal, which in turn is dependent upon the pulse power. If the pulse power is too low, the signal-to-noise ratio of the received signal will be degraded. If the pulse power is too high, the fiber response will be non-linear which also results in signal degradation.
As shown in prior art FIG. 1, DTS systems typically employ a Q-switched laser as the laser source. A Q-switched laser obtains high peak power, short duration laser pulses by controlling loop gain in the resonant cavity of the laser. A fast shutter is located between the active medium and the highly reflective mirror. The shutter is closed during pumping to reduce the loop gain to zero and prevent lasing. Since there is no lasing to deplete population inversion, energy stored in the active medium and amplifier gain both reach high values. The shutter is then opened producing a very high loop gain. The resulting high intensity standing wave utilizes the energy stored in the active medium to produce one giant pulse. The Q-switched laser is capable of producing extremely short, high energy output pulses at predictable times; however, it suffers from the following disadvantages. First, the pulse characteristics (width and power) of the pulse generated by the Q-switched laser are fixed by the design of the laser. Hence, a given DTS system will have a fixed spatial resolution. Furthermore, the optical power level of the pulse generated by the Q-switched laser may not be optimal for a particular installation. Second, the assembly of the Q-switched laser source involves the critical alignment of a complex optical assembly which must be maintained within tight tolerances over the lifetime of the system. These stringent requirements can result in relatively poor long term stability and reliability and increased manufacturing costs. Moreover, the parameters of the Q-switched laser tend to interact and thus degradation in one aspect (e.g., pulse power) tends to cause degradation in the others (e.g., pulse duration).
Thus, there remains a need for an improved pulsed-mode high power laser source suitable for use in DTS applications that provides for adjustable pulse characteristics (e.g., width and power), improved long term reliability, and reduced manufacturing costs.